Ultrasonic diagnostic apparatus for obtaining blood data

ABSTRACT

An interested region including an interested blood vessel of a subject is scanned by a plurality of ultrasonic waves so as to receive a plurality of echo signals. A plurality of Doppler signals is detected from the echo signals in connection with a plurality of sample points in the interested region. An average frequency of the blood current of the interested blood vessel, variance, and power are calculated based on the Doppler signals. A sample point positioned on the interested blood vessel is picked up from the plurality of samples every time phase based on the average frequency or power. A time curve is formed based on at least one of the average frequency of the picked up sample point, the variance, and power. Thereby, it is possible to compensate for the state that a sample volume is detached from the interested blood vessel as in the conventional case. After plural Doppler images were in a Doppler memory by `Freeze` operation, a ROI is set on the memorized images and a spectrum Doppler image for blood vessels in the ROI is formed. The ROIs are set on images while confirming moving blood vessels. A change of the velocity of the vessels is observed. Indexes such as an RI (Resistance Index) are calculated and help a diagnosis.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ultrasonic diagnostic apparatus forobtaining blood data by use of Doppler effect.

2. Discussion of the Background

FIG. 1 shows the structure of a conventional ultrasonic diagnosticapparatus corresponding to a spectrum Doppler mode. In this figure, apulse generator 121 generates a rate pulse at a period of a reciprocalnumber of a pulse repetition frequency (PRF). A pulser 103 generates avoltage pulse having a high frequency in synchronous with the ratepulse. A piezoelectric vibrator of a probe 101 is vibrated by thevoltage pulse. Thereby, an ultrasonic pulse is transmitted to a subject.A central frequency (transmission frequency) of the ultrasonic pulse isexpressed by c.

The ultrasonic pulse is reflected at a boundary of acoustic impedance ofthe subject, and the part of the reflected ultrasonic wave is returnedto the probe 101. Though the ultrasound is weak, the ultrasound scatterseven in blood corpuscles. Since the blood corpuscles are moved, thefrequency of the ultrasound is shifted in accordance with the velocityof the corpuscles. The spectrum Doppler mode observes the shiftedfrequency fd. The frequency fd can be obtained by the followingequation:

    fd=(2·V·fc·cos θ)/C

wherein a blood velocity is V, an angle between an ultrasonic beam and adirection of a blood current is θ, and a sound velocity of a living body(about 1530 m/sec) is C. The center frequency of the ultrasound is fc.

To obtain the shift frequency fd of the blood current, an echo signal isamplified by a preamplifier 105 and orthogonally detected through amixer 107 and a low pass filter 109. Thereby, a Doppler signalcorresponding to a shift frequency component can be obtained.

Then, the Doppler signal from the depth of a sample volume 101 istime-gated by a range gate 119. The gated Doppler signal is supplied toa fast Fourier transformer (FFT) 115 through a sample hold circuit 111,and a band pass filter 113. The FFT 115 Fourier transforms 128 Dopplersignals, which can be obtained by repeating transmission and receivingthe signals 128 times for the period of 1/PRF. Thereby, power for eachfrequency component, frequency spectrum, can be obtained. Such afrequency spectrum is arranged along a time axis as shown in FIG. 2, anddisplayed on a monitor 117 with brightness in accordance with power.Since such an image is often called a spectrum Doppler image, the namespectrum Doppler image is used hereinafter.

An observer can obtain various data from the spectrum Doppler image.There are indexes such as an RI (Resistance Index), a PI (PulsatilityIndex) other than information directly obtained from the Doppler image.For example, RI can be obtained by dividing a difference between amaximum velocity (maximum frequency) and a minimum velocity (minimumfrequency) in one cardiac cycle by the maximum velocity. PI can beobtained by dividing a difference between the maximum velocity and anaverage velocity (average frequency) in one cardiac cycle by the averagevelocity. Most of indexes can be calculated by substituting acharacteristic value extracted from the spectrum Doppler image for apredetermined equation.

However, the operation for calculating the indexes has the followingproblems:

(1) Since an operator must extract the characteristic value from thespectrum every time phase, a long processing time is required.

(2) The spectrum is lifted up by a contrast enhance effect of contrastenhance agent as shown in FIG. 3. As a result, the maximum frequency isshifted, and the value of the index is changed.

(3) As shown in FIG. 4, there is a case in which a sample volume isdetached from an interested blood vessel by influence of a motion suchas a breath motion, a pulsation, etc. In this case, the index isregarded as an error.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide an ultrasonicdiagnostic apparatus for forming a time curve of a blood current by anew method.

According to the present invention, an average frequency of a pluralityof sample points in an interested region is calculated. Then, a samplepoint positioned on the interested region is picked up from theplurality of the sample points based on the average frequency. As aresult, it is possible to compensate for the state that a sample volumeis detached from the interested blood vessel as in the conventionalcase.

Also, according to the present invention, there is used anauto-correlation method, thereby making it possible to form a time curveof a blood current obtained from an FFT Doppler waveform as in theconventional case.

Moreover, according to the present invention, a plurality of time curvesof the blood current at a plurality of sample points in the interestedregion are formed and displayed simultaneously, thereby making itpossible to obtain new and useful data.

Moreover, after plural Doppler images were in a Doppler memory by`Freeze` operation, an ROI is set on the memorized images and a spectrumDoppler image for blood vessels in the ROI is formed.

The ROIs are set on images while confirming moving blood vessels. Achange of the velocity of the vessels is observed. Indexes such as an RI(Resistance Index) are calculated and help a diagnosis.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention, in which:

FIG. 1 is a block diagram of a conventional ultrasonic diagnosticapparatus for a spectrum Doppler;

FIG. 2 is a view showing a conventional Doppler image;

FIG. 3 is a view showing a state that a spectrum is lifted up by acontrast enhance effect;

FIG. 4 is a view showing a state that a sample volume is detached froman interested blood vessel;

FIG. 5 is a block diagram of an ultrasonic diagnostic apparatus of thepresent invention;

FIG. 6 is a block diagram of a time curve unit of FIG. 5;

FIG. 7 is a view showing a scan region and a search region;

FIG. 8 is a specific view showing the search region;

FIG. 9A is a view showing one example of the time curve;

FIG. 9B is a view showing a display example of the time curve;

FIG. 10A is a view explaining a first receiving and transmitting methodfor achieving predetermined time resolution;

FIG. 10B is a view explaining a second receiving and transmitting methodfor achieving predetermined time resolution;

FIG. 11 is a view showing predetermined time resolution;

FIG. 12 is a time chart of the receiving and transmitting method of FIG.10B;

FIG. 13 is a view showing a time curve of e.g., average frequencyinterpolated by an interpolation section 51 of FIG. 6;

FIG. 14 is a view showing a modification of the ultrasonic diagnosticapparatus; and

FIG. 15 is a view showing the ROI set on the moving blood vessel.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will now be described withreference to the accompanying drawings.

FIG. 5 shows the structure of the ultrasonic diagnostic apparatus of thepresent invention. A probe 11 has a plurality of arrayed vibrators. Arate pulse generation circuit 13 generates a rate pulse in accordancewith a pulse repetition frequency PRF of e.g. 6 KHz. A transmissiondelay circuit 15 delays the rate pulse. A pulser 17 generates a voltagepulse having a frequency f₀ in synchronous with the delayed rate pulse.The voltage pulse vibrates the vibrators. Thereby, an ultrasonic pulsewhose central frequency is f₀ is transmitted to a subject.

The ultrasonic pulse is scattered at a boundary of acoustic impedance ofthe subject, and the part (echo) of the ultrasonic pulse is returned tothe probe 11. Information of the impedance difference is contained inpower of the echo. Velocity information of a moving object such as bloodis contained in the Doppler frequency of the echo.

The vibrators of the probe 11 convert the echoes to electrical signals.A preamplifier 19 amplifies the electrical signals. A receptiondelay/addition circuit 21 delays the amplified electrical signals, andadds up these signals. The added signal is referred to as an echosignal. In the echo signal, the echo component from the direction inaccordance with delay time is emphasized. The echo signal is sent to areceiver 23 and a color flow mapping unit (CFM unit) 31.

The receiver 23 logarithmically amplifies the echo signal, and detectsan envelope. A detected signal is sent to a monitor 29 through a B modememory 25 and a digital scan converter (DSC) 27 so as to be displayed asa B mode image (tissue tomographic image).

A CFM unit 31 comprises an orthogonal phase detector 32, ananalog-digital converter (ADC) 33, a buffer 35, a clutter filter 37, anauto-correlator 39, and a calculator 41. The orthogonal phase detector32 mixes a reference signal (f₀) and a reference signal (f₀) whose phaseis shifted at 90° with the echo signal, separately. As a result, aDoppler shift frequency component f_(d) and a high frequency component(2×f₀ +f_(d)) are extracted. Then, the orthogonal phase detector 32removes the high frequency component from each of the signals, so thattwo kinds of Doppler signals, each having only a Doppler shift frequencycomponent f.sub. d, are detected.

The analog-digital converter 33 samples the Doppler signals inaccordance with a predetermined sampling frequency. The sampled signalis sent to the clutter filter (MTI filter) 37. The clutter filter 37 hasa high-pass function, and removes the Doppler shift frequency component(low frequency component) of an internal organ whose motion isrelatively slow from each Doppler signal. Then, the clutter filter 37extracts a Doppler shift frequency component (high frequency component)of the blood current whose motion is relatively fast.

The auto-correlator 39 obtains the auto-correlation function of, e.g.,32 Doppler signals obtained for a period of 1/PRF every sample point. Anoutput of the clutter filter 37 is expressed by Z (i, Δt) and itscomplex conjugate is expressed by Z*(i, Δt). In this case, Δ is 1/PRFand i is a data number. If the number of data items of theauto-correlation calculation is M, the auto-correlation function C(τ)can be obtained by the following equation: ##EQU1##

The calculator 41 calculates the average frequency f_(average), varianceσ², and power P every sample point based on the auto-correlationfunction C(τ) by the following equation. ##EQU2## wherein Re(C(Δt)) is areal part, and Im(C(Δt) is an imaginary part.

As is known, in the calculation of the auto-correlation, the number ofdata items (observing time) may be about 1/10 as compared with FFT. Inother words, the average frequency of the large number of sample pointscan be calculated at a real time. Data of the average frequency is sentto the monitor 29 through a Doppler memory 43, and a digital scanconverter 27 so as to be displayed as a color blood current image.

A time curve unit 45 is supplied with data such as the average frequencyof the plurality of sample points in the search region from the Dopplermemory 43. As shown in FIG. 7, the search region is set on the B modeimage or the color blood current image to have an arbitrary size anddepth.

As shown in FIG. 8, the blood vessel does not stay at a fixed location,but moves by the breath motion and the pulsation. Due to this motion,the average frequency of the interested blood vessel cannot becontinuously observed at the same position (sample point). For thisreason, according to the present invention, the concept of the searchregion is introduced. The search region is set to surround the movingrange of the interested blood vessel. In other words, the size and thedepth of the search region are set such that the sample point, which ispositioned on the interested blood vessel, always exists in the searchregion.

As shown in FIG. 6, the time curve unit 45 comprises an interpolationsection 51, a search section 53, and a time curve forming section 55.The interpolation section 51 spatially interpolates data of e.g.,average frequency in the search region as required. The search section53 picks up the sample point positioned on the interested blood vesselfrom the plurality of sample points in the search region in accordancewith a predetermined rule to be described later.

Two kinds of rules are provided to pick up the sample point positionedon the interested blood vessel from the plurality of samples in thesearch region. Any one of the rules may be mounted. Or, both rules maybe mounted to be selectively used by the operator.

(First rule)

Power of all sample points in the search region is compared to selectthe maximum power. Then, the sample point showing the selected maximumpower is regarded as the sample point, which is positioned on theinterested blood vessel, to be picked up.

(Second rule)

The average frequencies of all sample points in the search region arecompared to select the maximum average frequency. Then, the sample pointshowing the selected maximum average frequency is regarded as the samplepoint, which is positioned on the interested blood vessel, to be pickedup.

The time curve forming section 55 forms various time curves of the bloodcurrent of the interested blood vessel by the average frequency,variance, and power of the sample point picked up by either rule. Thetime curve is displayed as a graph on the monitor 29 through the digitalscan converter 27.

As the time curves formed by the time curve forming section 55, as shownin FIG. 9A, there are the time curve of the average frequency (faverage)of the sample point picked up by the first or second rule, the timecurve of the power (maximum power) of the sample point picked up by thefirst rule, and the time curve of the maximum frequency of the samplepoint picked up by the first or second rule. In this case, the maximumfrequency f_(max) can be estimated from the average frequency faverageand variance σ by

    f.sub.max =f.sub.average +K·σ.

The time curve is not limited to the picked up sample point as shown inFIG. 9B. The time curves such as the average frequency of all samplepoints in the search region may be simultaneously displayed on the samescreen.

The simultaneous display can provide an image, which approximates theDoppler image due to the spectrum Doppler. Also, the simultaneousdisplay may provide new and useful data, which is different from theDoppler image.

Such a time curve may be described simply by a line. However, the timecurve may be displayed by various display methods. For example,brightness may be modulated in accordance with power, or color may bemodulated in accordance with variance.

After plural Doppler images were in a Doppler memory by `Freeze`operation, an ROI is set on the memorized images and a spectrum Dopplerimage for blood vessels in the ROI is formed.

An ROI is set to cover a blood vessel moving to every each frame asshown in FIG. 15. The time curves of the average frequency for aplurality of pixels in the ROI are displayed simultaneously whilemodulating brightness in accordance with the power for each pixel. Allkinds of indexes can be calculated from the spectrum Doppler image witha method as before.

According to the above-explained embodiment, the sample point positionedon the interested blood vessel moving by influence of the motion such asthe breath motion and the pulsation is searched. As a result, the timecurve of the average frequency can be obtained with high accuracy. Also,various indexes can be calculated with high accuracy from the timecurve.

Moreover, according to the above-explained embodiment, since the averagefrequency is used, unfavorable influence of contrast agent can bereduced. The average frequency can be calculated by use of the known CFMunit. As a result, there is no need of the conventional process forobtaining the average frequency from the frequency spectrum by thegravity calculation. In consideration of the process for obtaining theaverage frequency f_(average), only the auto-correlation function C (Δt)may be obtained. As a result, there may be set the number ofcalculations in which the number of multiplications is M and the numberof additions is M-1 when the number of data items is M. On the otherhand, in FFT, the number of calculations of (M·r)/2 is needed. In thiscase, M=2^(r). Moreover, in FFT, the following process is further neededto obtain the average frequency. ##EQU3##

In this case, S_(i) is a power spectrum of a frequency component f_(i).

The number of the auto-correlation calculation processes is smaller thanthe number of FFT calculation processes as shown by M/(M·r)/2)=1/4

wherein r=8, and M=258.

Generally, the number of data items of the auto-correlation is 32. Inconsideration of the number of data items, the number of theauto-correlation calculation processes is considerably smaller than thenumber of FFT calculation processes as shown by the following equation:

    M'/(M·r)/2)=1/32

In consideration of the process for calculating the average frequencyfrom the spectrum of FFT, the number of the auto-correlation calculationprocesses may be about 1/40 of the FFT.

Thus, as compared with the case in which the average frequency isobtained by FFT, the number of processes can be drastically reduced bydirectly obtaining the average frequency by the auto-correlation.

In the conventional spectrum Doppler, the echo may be received from onedirection passing through the sample volume. In the present invention,the echo must be received from a plurality of directions covering thesearch regions. Due to this, the reduction of time resolution may bebrought about. Specifically, necessary time resolution is, for example,100 Hz. In other words, the average frequency of each sample point inthe search region must be calculated every 1/100 seconds.

According to the present invention, time resolution of 100 Hz can berealized by a parallel simultaneous receiving technique and thelimitation of the scan range to the search region. In this case, as themain parameters of time resolution (frame rate), there are pulserepetition frequency PRF, the number of data items (observing time) ofauto-correlation process, the number of beam directions R covering thesearch region, and number of simultaneous receiving directions N (theecho is received from N direction simultaneously with one transmission).

Among these parameters, R is determined, depending on the size of thesearch region. N, which is necessary to achieve the frame rate of 100Hz, can be obtained by th e following equation:

    100 Hz=PRF/(M×(R·N))

For example, as shown in FIG. 11, it is assumed that PRF=6 kHz, thenumber of data times M=15, and R=16. N, which is necessary to achievethe frame rate of 100 Hz, is set to 4. FIG. 12 shows a time chart offour direction parallel simultaneous receivings. The number of beamdirections R=12.

Thus, the number of directions N of the simultaneous receivings iscalculated by a CPU 47 in accordance with the size of the search region.As a result, time resolution of 100 Hz can be achieved.

There is, of course, a limitation in the number of directions N of thesimultaneous receivings. Accordingly as the search region is enlarged,there may be a case in which the frame rate of 100 Hz cannot be achievedonly by the increase in N. In such a case, the pulse repetitionfrequency PRF is increased in accordance with the depth of the searchregion (depth of a visual field). Also, the beam pitch may be enlargedso that the number of directions R is reduced. A small number of dataitems M is effective for high frame rate. Moreover, as shown in FIG. 13,the frame rate of 100 Hz may be seemingly achieved by interpolation.

According to the present invention, as shown in FIG. 14, there can bealso used a spectrum Doppler circuit 50, which comprises a range gatecircuit 60, a sample hold circuit 61, a band pass filter 63, an analogdigital converter 65, and a fast Fourier transformer (FFT) 67. In thiscase, the auto-correlator 39 is connected to the analog digitalconverter 65 to input the shift frequency signal of the blood current.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative devices shown anddescribed herein. Accordingly, various modifications may be made withoutdeparting from the spirit or scope of the general inventive concept asdefined by the appended claims and their equivalents.

I claim:
 1. An ultrasonic diagnostic apparatus comprising:scanning meansfor scanning an interested region including an interested blood vesselof a subject so as to receive echo signals; means for detecting Dopplersignals from the echo signals for sample points in the interestedregion; means for calculating an average frequency, a variance, andpower at each of the sample points based on the Doppler signals; meansfor picking up out of the sample points phase, one sample point whoseone of the average frequency, the variance, and power is characteristicof the sample points; and means for forming a time curve of the bloodcurrent based on at least one of the average frequency, the variance andpower of the picked up sample point.
 2. The apparatus according to claim1, wherein said pick up means picks up the sample point showing maximumpower as a sample point positioned on the interested blood vessel. 3.The apparatus according to claim 1, wherein said pick up means picks upthe sample point showing a maximum average frequency as a sample pointpositioned on the interested blood vessel.
 4. The apparatus according toclaim 1, wherein said scanning means has means for forming the echosignals whose receiving directions are different from each other at onetransmission time.
 5. The apparatus according to claim 4, furthercomprising means for adjusting at least one of a pulse repetitionfrequency, a number of ultrasonic beam lines of said interested region,a number of receiving directions at one transmission and a number ofdata items of a frequency analysis so as to obtain said averagefrequency, the variance, and power on a predetermined time resolution.6. The apparatus according to claim 1, further comprising means for timeinterpolating said time curve.
 7. The apparatus according to claim 1,wherein the time curve is the time curve of the average frequency. 8.The apparatus according to claim 1, wherein the time curve is the timecurve of the power.
 9. The apparatus according to claim 1, furthercomprising means for estimating a maximum frequency based on the averagefrequency and the variance.
 10. The apparatus according to claim 9,wherein said time curve is the time curve of the maximum frequency. 11.An ultrasonic diagnostic apparatus comprising:means for scanning aregion of interest including an interested blood vessel of a subject byultrasonic waves, and wherein echo signals whose receiving directionsare different from each other are simultaneously generated with thescanning; means for adjusting the number of the echo signalssimultaneously generated in accordance with a size of the region ofinterest; means for detecting Doppler signals of a blood current of theinterested blood vessel from the echo signals; means for calculating anauto-correlation function of the Doppler signals; means for calculatingan average frequency, a variance, and power based on the calculatedauto-correlation function; and means for forming a time curve of theblood current based on at least one of the average frequency, thevariance and power.
 12. The apparatus according to claim 11, wherein thetime curve is the time curve of the average frequency.
 13. The apparatusaccording to claim 11, wherein the time curve is the time curve of thepower.
 14. The apparatus according to claim 11, further comprising meansfor estimating a maximum frequency based on the average frequency andthe variance.
 15. The apparatus according to claim 14, wherein the timecurve is the time curve of the maximum frequency.
 16. An ultrasonicdiagnostic apparatus comprising:means for scanning an interested regionincluding an interested blood vessel of a subject so as to receive echosignals; means for detecting Doppler signals from the echo signals for aplurality of sample points in the interested region; means forcalculating an average frequency, a variance, and power based on theDoppler signals; means for forming time curves of the blood currentbased on at least one of the average frequency, the variance and power;and means for simultaneously displaying the time curves for a pluralityof sample points.
 17. The apparatus according to claim 16, wherein saiddisplay means modulates brightness in accordance with the power for eachsample point.
 18. The apparatus according to claim 16, wherein the timecurve is the time curve of the average frequency.
 19. The apparatusaccording to claim 16, wherein the time curve is the time curve of thepower.
 20. The apparatus according to claim 16, further comprising meansfor estimating a maximum frequency of each sample point based on theaverage frequency and the variance.
 21. The apparatus according to claim20, wherein the time curve is the time curve of the maximum frequency ofeach sample point.
 22. A color Doppler apparatus comprising:means fordetecting a Doppler signal of each sample point in a set scanningregion; means for calculating an average frequency, a variance, andpower based on the Doppler signal at each sample point; means forstoring data of the average frequency, the variance and power of eachsample point over scanning of a plurality of frames; means for obtainingcolor Doppler images corresponding to the stored frames based on data onsaid memory; means for setting a region of interest having apredetermined size and a shape at a predetermined position on theobtained color Doppler image; means for forming a plurality of timechange curves of the sample points based on at least one of the storedaverage frequency, the variance, and power; and means for displaying thetime curves on a same screen.
 23. The apparatus according to claim 22,wherein said display means modulates brightness in accordance with thepower for each sample point.
 24. The apparatus according to claim 22,wherein the time curve is the time curve of the average frequency. 25.The apparatus according to claim 22, wherein the time curve is the timecurve of the power.
 26. The apparatus according to claim 22, furthercomprising means for estimating a maximum frequency of each sample pointbased on the average frequency and the variance.
 27. The apparatusaccording to claim 26, wherein the time curve is the time curve of themaximum frequency of each sample point.